This invention pertains to a method and apparatus for examining characteristics of materials on a molecular level using photoacoustic techniques, particularly, the examination of organic polymers.
Photoacoustic analytical techniques discussed herein are drawn from the infrared (IR) spectroscopic and acoustics arts. U.S. Pat. No. 4,255,971 to Rosencwaig, for example, discloses photoacoustic microscopy for examining materials on microscopic level. He also suggests the use of radiant energy, but not for rheo-photoacoustically examining molecular behavior. U.S. Pat. No. 4,875,175 to Egee, et al. also discloses photoacoustic spectroscopic techniques for analyzing thermal properties of layered materials. On the other hand, rheo-photoacoustical properties are measured in accordance with this invention.
It is also known, for instance, that molecular structures of certain chemical compounds produce characteristic energy absorption patterns in the IR spectrum due to their molecular vibration modes of the bonds that join the respective atoms. The molecules generally vibrate in the wave number range of 200 to 4000 cm.sup.-1. To identify the material, IR spectroscopy alone serves that purpose, if the material is IR transmissive, since each material exhibits a unique frequency IR absorption spectral pattern.
Rheological properties of optically transmissive polymers on a molecular level are described in "Rheo-optical Fourier-transform infrared (FT-IR) spectroscopy of polymers", Colloid & Polymer Science, Vol. 262, pp. 223-229, 1984, H. W. Siesler. Siesler, like many other prior infrared spectroscopy techniques, relates molecular events in the material under stress-strain conditions to IR intensities and/or IR band shift measurements.
On a microscopic level, as opposed to molecular level, properties of adhesive bonds or material flaws have also been examined. In the past, stress-wave emission techniques, such as disclosed by U.S. Pat. No. 4,004,456 to Vahaviolos, U.S. Pat. 4,100,808 to Evans et al., U.S. Pat. No. 4,501,149 to Konno, and U.S. Pat. No. 4,538,462 to Hartog were employed. From a rheological standpoint, though, optical systems have been limited to birefringence techniques, such as those described in U.S. Pat. No. 4,840,481 to Spillman, Jr. and U S. Pat. No. 4,777,358 to Nelson. No rheo-photoacoustical FT-IR techniques relating to vibrational modes of the substance under examination are believed to have been used in the past.